Today, at times, there is a need to prove that a particular version of a document existed on or before a particular time. In the past, this was accomplished by using a time stamp. Individuals looking to determine when a document might have existed would base their determination on the timestamp itself. However, for the determination to be accurate, the timestamp would have to be trustworthy.
In any system that incorporates logs (including event logs, version control changelogs, and workflow document logs), it is sometimes essential to be able to prove that such-and-such a log entry was made at a particular time (for example to prove that a particular version of a document was seen by so-and-so no later than now and no earlier than then). In other words, unforgeable timestamps are needed.
The usual way of dealing with this problem is to have a centralized “digital notary” service that dispenses unforgeable timestamps, which can then be applied to documents or log entries. This is done by hashing the entry, sending the hash to the notary, and receiving a verifiable receipt that can later be used to prove that the hash of the entry was seen by the notary at a particular time. From time to time, the notary service publishes its current “running hash” in a print medium such as a newspaper of record. Alternatively, a trusted source may supply certified, tamper-proof clock devices that issue digitally-signed certificates. It is essential in both centralized approaches that a service or device maintains a hash-connected log that can be used retrospectively to prove that a document with a given hash existed at a particular time.
Peer-to-peer systems, decentralized version-control systems like “git” and other decentralized systems make centralized methods impossible. There are a number of reasons for this. Separate devices have their own clocks, which may be inaccurate, and keep their own logs. They may be out of contact with a network for extended periods of time, and may often be turned off.
Public key cryptography has been used as a solution in the past. In such a case, public keys are used to authenticate. This allows a user to trust a particular server. However, a decision as to which server to trust must be made ahead of time.
NTP (Network Time Protocol) is used to synchronize clocks on networked devices. NTP involves synchronization messages between servers and clients. NTP version 4 includes a distributed security architecture called the Autokey public-key authentication protocol that allows the provenance of timestamps to be traced back to trustworthy sources to establish a trusted provenance for timestamps. However, it only allows the NTP client to accurately determine the current time. This by itself does not allow the accuracy of an NTP-generated timestamp to be verified at a later date.